It is well known that laser diodes alone produce a beam that is divergent and astigmatic. To get better performances from a laser diode, lenses can be placed in front of the beam emitted by the laser diode, to improve its performances.
Different types of lenses can be used to correct the divergence, symmetry and astigmatism of laser diodes. Because laser diodes have an elongated rectangular aperture through which the beam is emitted, the most widely used type of lenses are the cylindrical lenses.
Existing cylindrical lenses used for correcting laser diodes are made of a homogeneous medium, and have a cross-section either circular or noncircular. The cylindrical lenses of circular cross-section are easy to form, but they have poor optical performance when used at high numerical aperture, due to the large spherical aberrations. The cylindrical lenses of noncircular cross-section are capable of producing a better quality beam, but they are more difficult to produce since they require precision grinding of a relatively complex surface and precise centering of the two surfaces forming the lens. In use, the noncircular cylindrical lenses require precise positioning of the lens relative to the laser diode to obtain good results.
There are differenct types of lenses that have been termed as Luneburg lenses. The common threads for all of them are: the spherical symmetry (ball shape) or at least circular cross-section, the aberration-free imaging, except for chromatic aberration and field curvature, and the design principles where the graded index profile is calculated from pre-selected image and object positions. The main problem associated with the design of the Luneburg graded index lenses is to find the design whose refractive index distribution can be realized with the selected technology. With Luneburg cylindrical lens it is possible to preserve the circular cross section of the lens without introducing aberrations.
Known in the art are U.S. Pat. Nos. 5,080,706 and 5,155,631 (Snyder et al) which describe methods for fabrication of cylindrical microlenses of selected shape. These methods consist in first shaping a glass preform into a desired shape. Then, the preform is heated to the minimum drawing temperature and a fiber is drawn from it. The cross-sectional shape of the fiber is cut into sections of desired lengths. Finally, the fiber is cut into sections of desired lengths.
Also known in the art, is U.S. Pat. No. 5,181,224 (Snyder) which describes microlenses. This patent provides several microlens configurations for various types of optical corrections.
Another patent known in the art is U.S. Pat. No. 5,081,639 (Snyder et al), which describes a laser diode assembly including a cylindrical lens. This assembly comprises a laser diode and a cylindrical microlens whose cross-section is different from circular.